I encountered an issue with your code. I had to rename the file from R.lua to r.lua. It's also possible to write it like this if you want to keep R.lua:

require('neogen').setup({
  languages = {
    ['r'] = require('neogen.configurations.R')
  }
})

@KevinFalzone Can you please share how you have installed / are configuring neogen? Using Lazy.nvim I have the following (using my fork for now):

return {
  'RufusVijayaratnam/neogen',
    -- dir = vim.fn.expand("~/dev/neogen"),
    config = true,
    dependencies = { 'nvim-treesitter/nvim-treesitter', 'nvim-treesitter/playground' },
    cmd = "Neogen",
}

And this functions correctly if I open an R file an run the command :Neogen which results in:

#' @title
#' @description
#' @param x
#' @param y
#' @param ...
#' @return
#' @export
myfunc <- function(x, y, ...) {
    return(x + y)
}